Paper mills have for many years made extensive use, for the cleaning of paper making stock, of pressure screening apparatus embodying a cylindrical perforated screening member which defines screening and accepts chambers on the opposite sides thereof in a vertical housing, and wherein a rotor member operates in one of the chambers to keep the screening perforations open and free from solid material having a tendency to cling to the surface of the screening cylinder.
The assignee of this invention has manufactured and sold many such screens in accordance with a series of U.S. patents, commencing with Staege U.S. Pat. No. 2,347,716, and followed by Martindale U.S. Pat. No. 2,835,173 and numerous other patents including Seifert U.S. Pat. No. 3,849,302 and 4,105,543. In operation, the stock or furnish is delivered to the screening chamber adjacent one end of the screening cylinder, and the material rejected by the screening cylinder is collated and discharged from the opposite end of the screening chamber.
None of these types of screening apparatus have proved to be entirely satisfactory for screening stock which includes both heavy reject materials and also relatively large particles of reject materials whose specific gravities are essentially the same as or slightly less than water. The primary example of feed stock presenting this type of problem is wood pulp stock as it comes from a digester and therefore contains not only substantial quantities of pebbles and tramp metal, but also substantial quantities of knots whose specific gravity is close to that of water because they are impregnated with liquor absorbed during the digesting process.
If stock of this nature is treated in screening apparatus as outlined above wherein the screening chamber is inside the screening cylinder, the knots are likely to remain in the screening chamber for a substantial period and can effect considerable abrasion of the inner surface of the cylinder. The same is true to a lesser extent when the screening chamber is outside the screening cylinder, but it is still a problem to induce all of the knots to exit with the heavy reject materials from a reject outlet at the lower end of the screening chamber. Another problem is that in either of these cases, if the knots and heavy reject material are removed by way of the same reject outlet, a further operation is needed if it is desired to separate the heavy knots from the heavy reject for further processing in order to recover usable fiber therefrom.